Various instruments have been used on a commercial basis to test for both loss of metallic cross-sectional area and abrupt discontinuities in different types of ferromagnetic objects such as steel wire ropes. The testing of these ropes is particularly critical where the wire ropes are used in conveyer systems such as line shaft elevators and aerial cable systems used for transporting people from one region to another. The entire conveyer system is dependent upon the strength of the rope so that if the rope is weakened through loss of metallic cross-section area due to wear, corrosion or other effects or if it is weakened through broken wires or kinks. These weaknesses and the extent to which they effect the strength of the overall rope must be determined. If the rope is unduly weak at any point along its length it is removed from the conveyer system and this can only be determined through testing. The rope should not however be removed prematurely from the system because of the substantial replacement cost of the rope. Therefore any small weaknesses which are detected but which are not initially severe enough to justify removal and replacement of the rope should be monitored over time for increased weakening.
Steel wire ropes are only one example of the types of elongated ferromagnetic objects that should be subjected to these tests. For example there are many applications in which steel pipes, rods etc. should be tested for weakened regions to determine whether or not replacement is necessary.
Commercial instruments which have been available to date for testing ferromagnetic objects fall into two categories. The first category includes devices used primarily for measuring changes in metallic cross-sectional area of the object and the second category includes devices primarily used for detecting abrupt discontinuities in the mechanical structure of the object. Both types of devices which provide for non-destructive testing rely upon the magnetic permeability of the structure for setting up a uni-directional magnetic flux in the object. This flux is monitored for inconsistencies in the flux pattern which are representative of either changes of the cross-sectional area or anomalies in the object. In order to provide any meaningful and identifiable results from the testing the magnetic flux in the body must be substantial and will preferably saturate the body. This however presents difficulties from the standpoint that the instruments used must be capable of monitoring at an extremely high flux level, ie the level required to substantially saturate the body.
A structure which has been used in the past for testing of ferromagnetic bodies is described in Canadian Pat. No. 1,038,037 issued to Noranda Mines Limited on Sept. 5, 1978. According to the Noranda Patent an elongated ferromagnetic body is inserted in a pair of magnetic pole pieces of a testing device and a uni-directional magnetic flux which saturates the body flows through the section of the body between the pole pieces into the testing device to form a completed magnetic circuit. The pole pieces are provided with sensors which must be capable of sensing the main field of the high level magnetic flux to search for cross-sectional area differences in the body. A radial sensor is also provided for sensing radial components of the main field which are representative of anomalies such as abrupt discontinuities along the body.